The present invention generally relates to bearing lubrication and, more specifically, to an improved apparatus and method for providing lubrication to bearing supports in turbine engines.
Shaft-driven machinery, such as gas turbine machinery, typically include a centrally-located shaft mounted in support bearings rotating about an engine axis and housed in an engine casing. Lubrication, in the form of oil, is usually provided to the support bearings by means of an oil supply line provided to the engine casing, with the supply line usually attached to an internal lubricant inlet conduit connected to the bearing support. Scavenge oil may be removed from the bearing support and re-used after cooling and deaerating. A vent assembly may also be provided at the engine casing to remove air or an air/lubricant mixture from the bearing support.
During normal operation, the rotating shaft generates substantial heat which flows to the support bearings. The support bearings and the engine casing are further heated as additional thermal energy is generated by fuel that is consumed during turbine operation to produce a high-temperature gaseous flow stream. In addition to lubricating the support bearings, the process of circulating the oil serves to remove heat from the bearings so as to prevent overheating.
When the oil supply line is attached to an inlet conduit which is attached to the bearing support, a tight oil seal is formed and helps to prevent oil leakage into the turbine engine. However, as the turbine components and the inlet conduit expand and contract during normal operating conditions, this configuration produces stress and undesirable movement between the turbine components and the inlet conduit. This movement may result in leakage between the shaft, the bearing support, the inlet conduit, and the oil supply line.
One method to alleviate the problems resulting from high thermal gradients and associated thermal stresses is to use an o-ring configuration so as to allow limited movement while preventing oil leakage, as exemplified in U.S. Pat. No. 6,102,577 issued to Tremaine. The reference discloses a bearing gallery thermal movement isolation device comprising an o-ring disposed between an oil transfer tube and a sleeve to allow relative sliding motion while providing an oil-tight seal. However, the reference further discloses that, because the operating temperature of the bearing gallery may reach 375° F., use of a conventional o-ring material may result in failure of the oil pressure seal. Accordingly, the disclosed configuration requires the use of a specialized o-ring material.
In an alternative design configuration, a metal bellows is used to allow expansion and contraction while providing an air seal. FIG. 1 is an axial section view of a conventional turbine engine 10 illustrating a turbine bearing support assembly 20 with an internal rotating shaft 11. The shaft 11 is secured in a bearing support 13 which is disposed within an engine casing 15. Oil is supplied to the bearing support 13 via a lubricant inlet assembly 21 and an inlet conduit 23. A vent assembly 25 and a vent conduit 27 are provided as part of an internal pressure regulation system. A first scavenge port 31 and a second scavenge port 33 are provided for removal or circulation of the lubricant via a first scavenge conduit 35 and a second scavenge conduit 37, respectively. There may also be provided a buffer air port 39 and a buffer air conduit 41.
Thermal energy generated during normal operation produces elevated temperatures in the lubricant and in the various components comprising the turbine engine 10. The engine casing 15, for example, is directly exposed to hot gases or products of combustion, while the various conduits 23, 27, 35, 37, and 41 provide containment for the relatively cooler lubricant circulating through the bearing support 13. As noted above, temperature gradients are produced within the turbine engine 10 and cause different rates of expansion among the various engine components.
For example, when the turbine engine 10 is initially started, the temperature of the engine casing 15 may increase from ambient to as much as 1400° F., increasing at a rate different from the increase in temperature of the inlet conduit 23 which will remain relatively cooler than the engine casing 15. This process results in different rates of expansion and relative movement between the inlet conduit 23 and the surrounding structure. For example, initially the diameter of the engine casing 15 will increase while the length of the inlet conduit 23 will remain about the same. This will produce a movement between the engine casing 15 and an inlet receptacle 45, shown in FIG. 2.
Accordingly, in the present state of the art, the lubricant inlet assembly 21 may include a collar-like bellows 43 disposed between the inlet receptacle 45 and the inlet conduit 23. The bellows 43, which may be made of a thin sheet of metal alloy, provides a means of containing the hot gases while allowing for relative movement of the inlet conduit 23 and receptacle 45 as the turbine engine 10 continues to operate. This design, however, suffers from the shortcoming in that vibrational forces generated during normal operation cause cracks in the bellows 43 and result in air leakage.
As can be seen, there is a need for an improved apparatus and method that provides a closed lubrication system while operating in the demanding temperature environment of shaft-driven machinery.